1. Field of the Invention
The present invention relates to liquid crystal display device, and more particularly to a liquid crystal display device having an audio signal processing circuit formed of thin film semiconductor elements. In addition, the invention relates to an electronic apparatus using the liquid crystal display device having an audio signal processing circuit.
2. Description of the Related Art
In recent years, with the advancement of telecommunication technologies, mobile phones have been in widespread use. In future, transmission of moving images and a larger volume of information are expected. On the other hand, through reduction in weight of personal computers, those adapted for mobile communication have been produced. Information terminals called PDAs originated in electronic databooks have also been produced in large quantities and widely used. In addition, with the development of display devices, the majority of portable information devices are equipped with flat panel displays.
Among active matrix display devices, those adopting low-temperature polysilicon thin film transistors (thin film transistor is hereinafter referred to as TFT) are commercialized in recent years. By using the low-temperature polysilicon TFTs, not only pixels but also a signal line driver circuit can be integrated around the pixel portion, which contributes to downsizing or higher resolution of a display device. Therefore, it is further expected to be in wider use in future.
Meanwhile, as for portable information terminals, not only a visual display function but also other output functions such as an audio output function in particular are required. In displaying images, addition of an audio output to a visual output will bring about more satisfaction for a viewer. Thus, it is preferable that a display device has an audio output function.
Audio signals for an audio output can be classified into analog audio signals and digital audio signals. Among the analog audio signals, there are the one for directly obtaining a sound from a microphone and the like, and the one for obtaining a sound from an audio device of an analog output type. On the other hand, the digital audio signal is a signal for obtaining a sound from a digital audio device such as CD, MD and DVD players.
FIG. 2 is a block diagram showing a conversion of a digital audio signal into an analog audio signal. Most of CD players and the like are provided with an optical output which inputs an optical signal to an optical sensor 201 through an optical fiber. The optical sensor 201 converts the optical signal into an electric signal. This electric signal is specified by EIAJ (Electronic Industries Association of Japan) standard CP-1201 and the like. Then, the signal is decoded by a decoder 202 into serial digital audio data as shown in FIG. 3. The decoded signal is classified into three types: a base clock (BCL) of 1.411 MHz, an LR clock (LRCL) of 44.1 KHz and digital audio signal data (DATA). The digital audio signal data is sequentially transferred from the MSB (Most Significant Bit). The serial digital audio data is converted into parallel digital audio data by a serial-to-parallel converter circuit 203. Then, it is input to a resistor string D/A converter circuit 204 to be converted into an analog audio signal.
FIG. 5 is a conventional configuration of an audio signal processing circuit, which shows between a D/A converter circuit and a speaker connecting portion. The audio signal processing circuit includes a D/A converter circuit 501, an amplifier circuit 506, a speaker connecting portion 508, a power supply 502 for the D/A converter circuit 501, a switch 505 and an analog audio signal input terminal 507.
Operation of the circuit is described now. A parallel digital audio signal is converted into an analog audio signal in the D/A converter circuit 501 and then input to the switch 505. Meanwhile, an analog audio signal is input to the switch 505 from the analog audio signal input terminal 507. The switch 505 selects either of the output of the D/A converter circuit 501 or the analog input signal, and input it to the amplifier circuit 506. The amplifier circuit 506 amplifies the signal and output it to the speaker connecting portion 508.
A gain of the circuit is assumed to be as follows. Each output signal of the analog audio signal input terminal 507 and the D/A converter circuit 501 is at a level of 283 mVpp and 100 mVrms, and amplified 35.5 times as large in the amplifier circuit 506. That is, a signal at a level of 10 Vpp and 3.54 Vrms can be obtained at the speaker connecting portion 508.
As for the conventional audio signal processing circuit, there is the one including a D/A converter circuit with a power supply voltage of 3 V or 5 V and employing a resistor string as shown in FIG. 4. That is, a resistor string is constructed by connecting multiple resistors in series and a switch is disposed at each connecting node of the resistors, whereby the switch is turned ON/OFF to obtain a desired voltage (see Patent Document 1, for example).
[Patent Document 1] Japanese Patent Laid-Open No. 2000-138586
Operation of the circuit is described now with reference to FIG. 4. The description is given here using 3-bit signal data for simplicity, however, the actual D/A converter circuit for audio signals processes signal data of 10 bits or more in general. First, a high potential power supply is connected to a terminal VH. A low potential power supply is connected to a terminal VL. Between the terminals VH and VL, seven resistors 401 to 407 are connected in series. The resistors 401 to 407 are assumed to have the same resistance value. Assuming that the VL is at 2.3 V and VH is at 3.9 V, both ends of each resistor have a potential of 0.2 V. Then, switches 408 to 415 are connected at connecting nodes of the resistors 401 to 407. Furthermore, each pair of the switches 408 to 415, which are connected to on of the resistors 401 to 409 in common, is connected to switches 416 to 419 respectively. Then, the switches 416 and 417 are collectively connected to a switch 420, and similarly, the switches 418 and 419 are collectively connected to a switch 421.
Selection of the switches is controlled by a digital signal. That is, data of the LSB (Least Significant Bit) controls the switches 408 to 415, and then selects either of the switches 416, 417, 418 and 419. In addition, data of the second bit controls the switches 416 to 419, and selects either of the switches 420 and 421. In addition, data of the MSB (Most Significant Bit) controls either of the switches 420 and 421. In this manner, an output terminal 422 is connected to one of the connecting nodes of the resistors 410 to 407. For example, when the data of the LSB is 0, the switches 409, 411, 413 and 415 are turned ON while the switches 408, 410, 412 and 414 are turned ON when the data is 1. Similarly, when the data of the second bit is 0, the switches 417 and 419 are turned ON while the switches 416 and 418 are turned ON when the data is 1. When the data of the MSB is 0, the switch 421 is turned ON while the switch 420 is turned ON when the data is 1.
In the above case, when the digital audio data is 111, the switches 408, 416 and 420 are turned ON, and a voltage 3.9 V of the VH is output to the output terminal 422. Meanwhile when the data is 101, the switches 410, 417 and 420 are turned ON, and a voltage 3.5 V is output to the output terminal 422 through a buffer circuit 423. In this manner, a digital signal can be converted into an analog voltage.